Our work is focused on the molecular mechanisms responsible for establishing and maintaining stable states of gene expression during vertebrate embryogenesis. Progress has been achieved in three key areas: 1. We have further defined the interaction of transcription factors and linker histones with model nucleosomal structures including 5S ribosomal RNA genes. These results demonstrate that post-translational modifications of core histones can facilitate transcription factor association and impede the correct association of linker histone. Nucleosomes are dynamic structures that can respond to external cues. 2. We have established RNA polymerase II transcription extracts from Xenopus oocytes, eggs and somatic cells. These have been used to examine: a) the developmental regulation of gene normally expressed only in gametes, eg. hsp7O, FRG Y2, TFIIIA or only in differentiated somatic cells, eg histone H1degree; b) the influence of chromatin structure on tissue specific gene expression. Developmentally regulated transcription factors have been identified and nucleosome assembly found to have a positive role in the control by the sex-hormone estrogen of vitellogenin gene transcription. 3. Germ cell selective variants of the Y-box transcription factors have been found to store mRNA in Xenopus oocytes and mammalian spermatocytes. These are the first proteins 'masking' mRNA to have been characterized at the molecular level.